Highlight unselected options

Could you please check after adding the below code in your child theme’s functions.php file.

add_action( 'wp_footer', 'highlight_and_scroll_unselected_variation_labels', 100 );
function highlight_and_scroll_unselected_variation_labels() {
    if ( ! is_product() ) return;
    ?>
    <script type="text/javascript">
        jQuery(function($) {
            var $form = $('form.variations_form');

            $form.on('click', '.single_add_to_cart_button', function(e) {
                var isValid = true;
                var firstInvalid = null;

                $form.find('.variations select').each(function(){
                    var $select = $(this);
                    var selected = $select.val();
                    var $label = $select.closest('tr').find('label');

                    if (!selected) {
                        isValid = false;
                        $label.css('color', 'red');
                        if (!firstInvalid) {
                            firstInvalid = $label;
                        }
                    } else {
                        $label.css('color', '');
                    }
                });

                if (!isValid) {
                    e.preventDefault();

                    if (firstInvalid) {
                        $('html, body').animate({
                            scrollTop: firstInvalid.offset().top - 100 // adjust offset as needed
                        }, 500);
                    }

                    return false;
                }
            });

            // Reset label color on change
            $form.on('change', '.variations select', function(){
                var $select = $(this);
                var $label = $select.closest('tr').find('label');

                if ($select.val()) {
                    $label.css('color', '');
                }
            });

            // Block default alert
            if (typeof window.alert === 'function') {
                const originalAlert = window.alert;
                window.alert = function(msg) {
                    if (msg === wc_add_to_cart_variation_params.i18n_make_a_selection_text) {
                        return;
                    } else {
                        originalAlert(msg);
                    }
                };
            }
        });
    </script>
    <?php
}

Kindly check and verify it from your end?

Have a great day!